In the so-called Mannesmann tube making process used as the typical method of producing the seamless tube, a billet heated to a predetermined temperature is fed to a piercer, and a hollow shell is produced by piercing an axial center portion of the solid billet. Then, the pierced hollow shell is passed through an elongating mill (mandrel mill) composed of five to eight stands to undergo a process of reducing the wall thickness of the hollow shell. After or without re-heating the hollow shell, correcting in the form and sizing are performed by a stretch reducing mill or a sizing mill. Then, a finishing process is performed to the hollow shell to obtain the seamless tube which is of a product.
In the rolling with the piercer, inclined rolls oppose to each other with respect to a pass line such that the billet which is of a material to be rolled is moved in a rolling direction along the pass line. A plug is also positioned between the inclined rolls, and the plug is held by a mandrel bar arranged on the pass line.
FIG. 1 is a view schematically explaining an arrangement of the inclined rolls used in piercing. FIG. 2 is a view explaining the arrangement of the inclined roll, seen from the direction of an arrow A-A shown in FIG. 1.
As shown in FIG. 1, inclined rolls 1 are arranged in an axisymmetrical manner such that roll axis lines respectively form cross angles γ with respect to a pass line X-X. As shown in FIG. 2, the inclined roll 1 is arranged so as to form a feed angle β with respect to the pass line X-X. On the other hand, the other inclined roll 1 (not shown in FIG. 2) is also arranged so as to be reversely inclined with the feed angle β across the pass line X-X.
The inclined rolls 1 applying screwing movement to a billet 3 are directly coupled with drive devices 4 respectively, and thereby the inclined roll 1 can be rotated about the roll axis line while the cross angle γ and the feed angle β are separately maintained. Disk rolls 5 serving for a tube material guide are also arranged between the inclined rolls 1 and 1 as opposed to each other, wherein the disk rolls 5 oppose to each other with respect to the pass line X-X while a phase of the disk rolls 5 differs from that of the inclined rolls by 90°. In FIG. 2, the disk rolls 5 are shown by an alternate long and short dash line as an imaginary line. An end portion of a plug 2 is supported by a front end of a mandrel bar M, and the plug 2 is arranged on the pass line X-X as a tool for piercing/rolling. In the piercer having the above configuration, during the period when the billet 3 fed in the direction of an outline arrow sign on the pass line X-X is passed through a gap between the inclined rolls, the part of the billet 3 at the instant passing stage is pierced into a hollow part while the process of reducing the wall thickness thereof is also performed by the inclined rolls 1 and the plug 2, whereby the billet 3 is moved forward along the pass line X-X while revolving, and eventually the plug 2 makes an entire hole in the axial center portion of the billet 3 to yield the hollow shell.
FIG. 3 is a view showing an outer contour profile in a longitudinal direction of the plug adopted as the tool for piercing/rolling. Usually, the plug 2 includes a rolling section, a reeling section, and a relief section, and the plug 2 is formed in a cannon ball shape in which the rolling section has a sharp-nosed leading edge portion.
A Cr—Ni low alloy steel is usually used as a plug material of the tool for piercing/rolling. In order to obtain heat insulating effects and lubricating effects in the piercing, before using the plug 2, heat treatment is performed to the plug in an oxidizing atmosphere to form an oxide film on a surface of the plug 2. A thickness of the oxide film ranges from 100 to 1000 μm, and the oxide film is mainly composed of iron oxide.
However, as shown in FIG. 3, in the sharp-nosed front end portion of the rolling section of the plug 2 used as the tool for piercing/rolling, a volume is small, and a temperature rapidly rises by heat generation of the material to be rolled in association with the piercing. In the case where base material strength of the plug cannot withstand a load of heat, dissolution wastage is generated in the front end portion of the plug.
When the piercing is performed with the plug in which the dissolution-induced metal loss is generated in the front end portion, the inside surface defects are generated in the hollow shell, which results in a large problem in quality. When a degree of the generated dissolution wastage is increased, it is necessary to interrupt the rolling in the way through the piercing process, which results in a significant decrease in productivity.
The following is the durability of the plug. When the material to be rolled is made of carbon steel, the plug can withstand the piercing runs for more than 100 passes. However, when the material to be rolled is made of stainless steel or high alloy steel, it is necessary to scrap the plug after several passes. Usually the damage which is judged as the end of a life-time of the plug is concentrated on the plug front part. The plug whose life-time is ended is recycled by remachining the plug to an extent in which a trouble is not generated in the mandrel bar holding the plug. When the plug exceeds the remachining range, the plug is scrapped.
Therefore, the life-time of plug has a large influence on production cost of the seamless tube. Particularly, with deep drilling of an oil well or development of oil well in the sea bottom in recent years, an expense ratio of the plug tool cost rises more and more in the production cost of the seamless tube by increasing needs for stainless steel or high alloy steel in which the impact on the plug is increased in the piercing.
In order to achieve the extension of the life-time of plug, there are various proposals. For example, Japanese Patent Application Publication No. 7-60314 proposes a plug which is made of Cr—Ni low alloy steel to form the thick oxide film on the plug surface. Elements such as W, Mo, Nb, Ti, and Nb are added to the Cr—Ni low alloy steel in order to form the oxide film which has excellent adhesion to the base metal.
However, when the piercing is performed with the plug proposed in Japanese Patent Application Publication No. 7-60314, the oxide film on the plug partially comes off to degrade surface quality of the plug. When the piercing is performed with the plug whose surface quality becomes degraded, the surface quality of the plug is printed to an inner surface of the material to be rolled, and the surface quality is degraded in the inner surface of the hollow shell after the rolling. When finish rolling is further performed to the hollow shell with a mill, many minute seam defects in a rice grain form are generated in the inner surface of the mother pipe after the final finishing rolling.
Japanese Patent Application Publication No. 10-249412 proposes a piercing/rolling plug in which the plug reeling section is smaller than the rolling section in the thickness of oxide film. In the production of the plug proposed in Japanese Patent Application Publication No. 10-249412, the thick film is evenly formed on the plug surface, and the film of the reeling section is mechanically ground to decrease the film thickness. In grinding the film of the reeling section, it is necessary to strictly manage a grinding amount.
However, before forming the film, frequently the plug shape is not formed in a perfect circle. Therefore, it is difficult that the film thickness is strictly adjusted in a circumferential direction of the plug, and the damage is generated in a plug region where the film thickness is not more than a necessary thickness.
Japanese Patent Application Publication No. 2002-113507 proposes a rolling plug which includes a coated layer on an outer surface, wherein the coated layer is made of a niobium-base alloy having predetermined resistance to compressive deformation, and the niobium-base alloy contains Ti in the range of 7 to 45 mass %. Japanese Patent Application Publication No. 6-328105 proposes a tool for rolling/piercing in which the coated layer containing Mo, Ni, and Cr is formed by overlay welding. Japanese Patent Application Publication No. 2-63604 proposes a plug in which a portion coming into contact with the material to be rolled is formed by a porous dispersed layer and a continuum phase having a melting point lower than that of the porous dispersed layer. The porous dispersed layer is formed by Mo-base alloy powders.
However, when the plugs proposed in Japanese Patent Application Publication Nos. 2002-113507, 6-328105, and 2-63604 are adopted, unit cost of the plug becomes expensive compared with the Cr—Ni low alloy steel as describe later, and the expense ratio of the plug tool cost is further increased in the production cost of the seamless tube.